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Extreme heat during bud differentiation can lead to double cherries or spurs, where one side of the cherry is aborted. Photo by Tim Smith, WSU Extension

Extreme heat during bud differentiation can lead to double cherries or spurs, where one side of the cherry is aborted. Photo by Tim Smith, WSU Extension

Cherry doubling is a sporadic problem but one that costs the Pacific Northwest cherry industry millions of dollars when it happens.

Doubled cherries, which are usually treated as culls, form when the flower buds are damaged by excessive summer heat.

Tim Smith, Washington State University Extension educator for north central Washington, told growers during Stone Fruit Day that misinformation has been circulated about doubling. It is not caused by chemical sprays, such as dimethoate, after harvest. “It absolutely, positively, is not,” he stressed.

When flower buds for the following year are developing, excessive heat can cause the ovule to double, resulting in a double cherry or a spur, where one side of the fruit is aborted.

The doubling can be seen in the spring, as soon as the fruit starts to develop, but by that time it is almost a year since the damage was done, Smith said.

Orchards at high elevations are unlikely to be concerned with doubling, but it can be a frequent and serious problem in the earlier growing regions of Washington, where summer temperatures are higher, he said. In some orchards, it is a worse problem than bird damage.

It is most likely to occur when temperatures exceed 100°F, and the more days the weather stays hot, the worse it is likely to be. If the weather is relatively cool in July, very little doubling is likely the following season.

As the cherry industry shifts from large, shaded trees to smaller trees with greater exposure to the sun, it becomes more important to address the issue, Smith said.

Cooling

He believes the disorder can be managed by using overtree cooling to reduce the temperature in the orchard during the summer. The critical period is probably the last three weeks in July, or perhaps a week earlier in the warmer parts of Washington, he said.

To protect cherries, cooling isn’t needed all summer long, like it might be to protect apples from sunburn.

“Cooling a cherry orchard is going to be a much easier job,” Smith said. “You need far less water, and fewer days of cooling, to make a difference.”

Field trial

Smith conducted a field trial in a Bing orchard in the Tri-Cities area of Washington in 2004. In an untreated drip-irrigated block with no ground cover, which was used as a check, 24 percent of the fruit were spurs or doubles. Had the block been in a different area, it probably wouldn’t have been picked, but because it was in an early area and the cherries sold for a high price, it was economically feasible to sort out the doubles.

In another block of the orchard, which had undertree sprinklers, 12 percent of the fruit was doubled. Smith said this block was not intentionally cooled, but the undertree irrigation and grass on the orchard floor had some cooling effect.

In a mature block with overtree sprinklers that was cooled continuously when the temperature was above 90°F, less than 3 percent of the fruit were doubles, whereas the block usually had more than 15 percent without cooling.

In a fourth block, where overtree cooling was cycled for 30 minutes on and 30 minutes off during hot periods, doubling was less than 2 percent.

Smith estimates that the orchard owners cooled on 12 to 15 days, and cooling reduced overall cullage by 10 percent on the 175 acres of cherries. The economic benefit was probably 20 to 100 times the cost of cooling, he said. “On a hot summer day, these guys are busy now, and they’re busy making a lot of money the next year.”

Research

Smith said the trial showed that cooling can reduce doubling, but more information is needed about exactly when to begin cooling and when to stop, and about the critical temperatures and timing for varieties other than Bing.

Dr. Matt Whiting, horticulturist with Washington State University, is conducting research to pinpoint the exact temperatures and timing when cooling might be necessary.

Some growers resist the idea of cooling the orchard after harvest, Smith acknowledged. For one thing, it’s supposed to be a time to slow down and take some time off. There are also concerns that applying additional water will lead to outbreaks of diseases, such as mildew.

Smith said the mildew fungus doesn’t like to be wet and is no more of a problem in orchards with overtree sprinkler systems than with undertree irrigation. For cooling, a high-power overtree impact system might not be necessary, and a system that uses less water and is easier to install—such as a mister system—might do the job.

As cherry volume increases in the Pacific Northwest, packers might be reluctant to handle cherry lots with a high percentage of cullage, Smith said, and cooling might be important for more than just improving returns.